A waste heat steam generator is a heat exchanger which recovers heat from a hot gas flow. Waste heat steam generators are frequently used in combined cycle power plants which predominantly serve to generate electricity. In such cases a modern combined cycle power plant usually comprises between one and four gas turbine and at least one steam turbine, with each turbine either driving a generator (multi-shaft plant) or a gas turbine with the steam turbine on a common shaft driving a single generator (single-shaft plant). The hot exhaust gases of the gas turbine are used in such plants in a waste heat steam generator for generating water steam. The steam is subsequently supplied to the steam turbine. Usually around two thirds of the electrical power is allocated to the gas turbine and a third to the steam process.
Like the various pressure stages of a steam turbine, the waste heat steam generator also comprises a plurality of pressure stages with different thermal states of the water-steam mixture contained therein in each case. In each of these pressure stages the flow medium on its flow path initially passes through economizers, which use the residual heat for preheating the flow medium and subsequently different stages of evaporator and superheater heating surfaces. In the evaporator the flow medium is evaporated, thereafter possible residual moisture is separated off in a separation device and the steam left behind is heated up further in the superheater.
Because of fluctuations in load the heat power transferred to the superheater can be heavily influenced. Therefore it is frequently necessary to regulate the superheating temperature. With new plants this is mostly achieved by an injection of feed water between the superheating surface for cooling, i.e. an overflow line branches from the main flow of the flow medium and leads to injection valves disposed there accordingly. The injection is regulated in this case via the exit temperatures at the respective superheaters.
Modern power plants not only demand high levels of efficiency but also a method of operation that is as flexible as possible. As well as short startup times and high load change speeds, these also include the option of compensating for frequency faults in the electricity grid. In order to fulfill these requirements the power plant must be able to provide additional power of for example 5% and more within a few seconds.
This is realized in previous usual combined cycle power plants by a load increase of the gas turbines. Under certain circumstances it can however be possible in the upper load range that the desired power increase is not able to be exclusively provided by the gas turbines. Therefore in the interim solutions have also been pursued in which the steam turbine can and should also make an additional contribution to frequency support in the first seconds.
This can be done for example by opening partly throttled turbine valves of the steam turbine or what is referred to as a step valve, which reduce the steam pressure of the steam turbine. Steam from the boiler of the upstream waste heat steam generator is released by this process and supplied to the steam turbine. With this measure a power increase in the steam part of the combined cycle power plant is achieved within a few seconds.
This additional power can be released in a relatively short time, so that the delayed power increase by the gas turbines (restricted by a maximum load change speed resulting from mechanical and operational conditions) can be compensated for at least partly. The entire block immediately makes a jump in performance through this measure and can through a subsequent increase in power of the gas turbine also maintain this performance level permanently or exceed it, provided the plant was in part load mode at the time that the additional power reserves were requested.
A permanent throttling of the turbine valves to maintain a reserve however always leads to a loss of efficiency, so that to drive the system cost effectively the degree of throttling should be kept as low as is absolutely necessary. In addition a number of designs of waste heat steam generators, for example once-through steam generators under some circumstances demand a significantly lower boiler volume than for example natural boiler steam generators. The difference in the size of the reservoir has an influence in the method described above on the behavior during changes in power of the steam part of the combined cycle power plant.